Measuring devices which are implemented for progressive tracking of a target point and a coordinate position determination of this point can generally, in particular in conjunction with industrial surveying, be summarized under the term laser trackers. A target point can be represented in this case by a retroreflective unit (for example, a cube prism), which is targeted using an optical measurement beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is detected using a detection unit of the device. An emission or reception direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the detection of the beam, for example, by means of runtime or phase difference measurement.
Laser trackers according to the prior art can additionally be embodied having an optical image detection unit having a two-dimensional, light-sensitive array, for example, a CCD or CID camera or a camera based on a CMOS array, or having a pixel array sensor and having an image processing unit. The laser tracker and the camera can be installed one on top of another in this case in particular in such a manner that the positions thereof in relation to one another are not variable. The camera is, for example, rotatable together with the laser tracker about its essentially perpendicular axis, but is pivotable up-and-down independently of the laser tracker and is therefore arranged separately from the optic of the laser beam in particular. Furthermore, the camera—for example, as a function of the respective application—can be embodied as pivotable about only one axis. In alternative embodiments, the camera can be installed in an integrated construction together with the laser optic in a shared housing.
With the detection and analysis of an image—by means of image detection and image processing unit—of a so-called measuring aid instrument having markings, the relative locations of which to one another are known, an orientation of the instrument and of an object (for example, a probe), which is arranged on the measuring aid instrument, in space can be concluded. Together with the determined spatial position of the target point, furthermore the position and orientation of the object in space can be precisely determined absolutely and/or in relation to the laser tracker.
Such measuring aid instruments can be embodied by so-called scanning tools, which are positioned having the contact point thereof on a point of the target object. The scanning tool has markings, for example, light spots, and a reflector, which represents a target point on the scanning tool and can be targeted using the laser beam of the tracker, wherein the positions of the markings and the reflector in relation to the contact point of the scanning tool are precisely known. The measuring aid instrument can also be, in a way known to a person skilled in the art, a handheld scanner equipped for distance measurement, for example, for contactless surface surveying, wherein the direction and position of the scanner measurement beam used for the distance measurement are precisely known in relation to the light spots and reflectors which are arranged on the scanner. Such a scanner is described, for example, in EP 0 553 266.
In addition, in modern tracker systems, a deviation of the received measurement beam from a so-called servo-monitoring point is ascertained on a sensor—increasingly as a standard feature. By means of this measurable deviation, a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined and the alignment of the laser beam can be corrected or tracked as a function of this deviation such that the deviation on the sensor is decreased, in particular is “zero”, and therefore the beam is aligned in the direction of the reflector center. By way of the tracking of the laser beam alignment, progressive target tracking (tracking) of the target point can be performed and the distance and position of the target point can be progressively determined in relation to the measuring device. The tracking can be implemented in this case, for example, by means of an alignment change of the deflection mirror, which is movable by a motor and provided for deflecting the laser beam, and/or by a pivot of the targeting unit, which has the beam-guiding laser optic.
The described target tracking must be preceded by locking of the laser beam on the reflector. For this purpose, a detection unit having a position-sensitive sensor and having a comparatively large field of vision can also be arranged on the tracker. Moreover, in devices of the type in question, additional illumination means are integrated, using which the target or the reflector is illuminated, in particular using a defined wavelength differing from the wavelength of the distance measuring means. The sensor can be implemented in this context to be sensitive to a range around this specific wavelength, for example, to reduce or entirely prevent external light influences. By means of the illumination means, the target can be illuminated and, using the camera, an image of the target having illuminated reflector can be detected. By way of the imaging of the specific (wavelength-specific) reflection on the sensor, the reflection position in the image can be resolved and therefore an angle in relation to the detection direction of the camera and a direction to the target or reflector can be determined. An embodiment of a laser tracker having such a target search unit is known, for example, from WO 2010/148525 A1. In dependence on the direction information thus derivable, the alignment of the measurement laser beam can be changed such that a distance between the laser beam and the reflector, onto which the laser beam is to be locked, is decreased.
Laser trackers of the prior art have at least one distance meter for distance measurement, wherein it can be implemented as an interferometer, for example. Since such distance measuring units can only measure relative distance changes, so-called absolute distance meters are installed in addition to interferometers in current laser trackers. For example, such a combination of measuring means for distance determination is known by way of the product AT901 of Leica Geosystems AG. The interferometers used in this context for distance measurement primarily use gas lasers—as a result of the long coherence length and the measurement range thus made possible—as light sources, in particular HeNe gas lasers. The coherence length of the HeNe lasers can be several hundred meters in this case, so that the ranges required in industrial metrology can be achieved using relatively simple interferometer constructions. A combination of an absolute distance meter and an interferometer for distance determination using an HeNe laser is known, for example, from WO 2007/079600 A1.
By way of the use of such an interferometer for distance determination or determination of the distance change in a laser tracker, a very high measurement precision can be implemented as a result of the interferometric measuring method thus usable.
To ensure high measurement precisions, the tracker system must be calibrated before executing measurements. Such a calibration can, on the one hand, be carried out before delivery of the device, so that, for example, an offset of a target axis predefined by the structural construction of the tracker in relation to an emission axis along which the measurement radiation is emitted is previously known and is taken into consideration as measurements are carried out. On the other hand, a recalibration can and must be performed after a specific operating time of the system, since errors can occur in the system, for example, due to environmental influences (for example, temperature and/or pressure variations) or mechanical effects (for example, shocks).
In addition, fine calibration is to be provided at shorter intervals (for example, every time it is put into operation) with respect to very sensitive components or in the event of a relatively intensive stress of the measuring device, to continuously and progressively maintain required precisions.
Such a calibration can be provided in particular by multiple two-location measurements having significant distance differences to one or more separate reference targets, which can be freely offset in space. Calibration values for, e.g., a PSD offset, a target axis distance (=parallel offset), and/or a target axis error (=directional error) can be derived therefrom. Such a calibration is provided, for example, for the laser trackers “AT901” and “AT401” of Leica Geosystems AG.
This conventional procedure for calibration is contradictory in particular to a general requirement that all required components for calibration are to be integrated in the tracker system or permanently attached thereon.
In addition, such a method may only be carried out without user intervention when the measuring system is located in a fixed installation, in which a remote reflector can be permanently installed, for example, on an opposing building wall. However, for an application of a laser tracker as a mobile measuring system in changing surroundings, such a calibration using positionally fixed retroreflectors cannot be carried out.
To overcome these obstructions, U.S. Pat. No. 7,327,446 B2 discloses an arrangement of two different reflective objects on a laser tracker for its calibration. One of these objects is embodied as a cube prism and the second as a planar mirror, wherein self-calibration of the tracker can be carried out by targeting both the cube prism and also the planar mirror.
Especially due to the requirement that the components (mirror and prism) required for the self-compensation are located in or on the tracker, the calibration measurements can only be executed at essentially equal and relatively short distance, however. In particular, calibration of the system with respect to significantly different distances and in particular with respect to comparatively great distances (at least several meters), which occur during a typical use of such a tracker, thus cannot be executed. This represents a main disadvantage of this embodiment with respect to the most robust possible calibration, which is therefore valid and reliable for the measurement range to be covered. For typical calibration methods—as mentioned above—the necessity of a user intervention for the calibration is also to be noted as the main disadvantage.